Air-assisted and air driven fuel injectors have been proposed to produce a finely atomised spray of fuel. However, such injectors require a source of compressed gas. Previously, mechanically driven compressors were used for this purpose but apart from their additional cost such compressors set a lower limit on the idling speed because the air used for atomisation bypassed the intake throttle.
To mitigate the foregoing disadvantages, a compressed gas supply in an internal combustion engine has been proposed in EP-A-315,328 that comprises a one-way valve in at least one of the combustion chambers of the engine for bleeding a small proportion of the compressed charge into a plenum chamber. The plenum chamber has an outlet from which gases can be supplied at a pressure substantially lower that the pressure within the plenum chamber.
It is desirable in such a engine to provide a regulating valve that can allow a gas flow regulation dependent on the temperature of the engine. One should draw a greater mass of gas during cold starts and warming up of the engine to improve atomisation. However, when the engine is warm, a lesser mass of gas is sufficient to achieve good mixture preparation and continuing to extract gas at the same rate as during cold operation would be wasteful of energy. The provision of a regulating valve that can vary with engine temperature in this way adds to the complexity and the cost of the regulating valve.
According to the present invention, there is provided a compressed gas supply in an internal combustion engine, comprising a one-way valve in at least one of the combustion chambers of the engine for bleeding a small proportion of the charge compressed in the combustion chamber into a plenum chamber and a tube communicating at one end with the plenum chamber and having at its other end an outlet from which gases can be supplied at a pressure substantially lower than the pressure within the plenum chamber, wherein the tube is a capillary tube that acts as a temperature dependent flow regulator.
The invention therefore is based on the use of a capillary tube to regulate the pressure drawn from the plenum chamber, which is at a pressure substantially equal to the peak pressure in the combustion chambers. Such a means of dropping the pressure from typically 13,680 KPa (2000 psi) to between 103 and 513 KPa (15 and 75 psi), has the advantage of not using any moving parts and automatically permitting higher gas flow rates when the engine is cold because the viscosity of the gases flowing through the cold capillary tube is lower.
To maintain the capillary at the same temperature as the engine, it can be heated by the water jacket of the engine but the same effect can be achieved by forming the plenum chamber within the body of the engine and housing the capillary tube within the plenum chamber.
There is always a pressure drop across a tube that depends on the diameter of the tube, its length and the viscosity of the fluid flowing through it. One can achieve the same pressure drop either by reducing the diameter or extending the length of the tube. In the present invention, a wide range of tube lengths and diameters can achieve the desired effect but other factors place limits on these dimensions. In particular, a very narrow tube could easily be blocked and it would be difficult to calibrate the tube to give identical flow for different engines, taking into account the inevitable manufacturing tolerances. A very long and relatively wide tube on the other hand is difficult to package within the engine.
To meet these conflicting requirements, it is desirable to form the capillary tube as a coil thereby permitting a long tube of sufficient diameter to be wrapped into the space available within the plenum chamber.
The tube may be of uniform diameter throughout its length but alternatively it is possible for it to be stepped to permit a gradual transition towards the end connected to the supply outlet.